Infra-red image intensifier having a tunnel-emission cathode having a conductive mosaic



Feb. 6, 1968 Y KAZAN 3,368,077

INFRA-RED IMAGE INTENSIFIER HAVING A TUNNEL-EMISSION CATHODE HAVING ACONDUGTIVE MOSAIC Filed March 8, 1963 EMHTED EMTTED \NCIDENT TUNNEL EggL!6HT RADIATKON ELECTRONS 55 BENJAM/N KAZA/V INVENTOR.

United States Patent O 3,368,077 INFRA-RED IMAGE INTENSIFIER HAVING ATUNNEL-EMISSION CATHODE HAVING A CONDUCTIVE MOSAIC Benjamin Kazan, LosAngeles, Calif., assignor to Electro- Optical Systems, Inc., Pasadena,Calif. Filed Mar. 8, 1963, Ser. No. 263,861 3 Claims. (Cl. 250-213) Thepresent invention relates in general to the image intensifier tube artand more particularly relates to a new and improved image intensifiertube based on what are known as cold emission techniques.

With respect to the conventional image intensifier, an image of visiblelight falling on the photoemissive surface causes the emission of apattern of electrons. These are accelerated and imaged on a phosphorscreen to provide an intensified image corresponding to the input image.However, to a large extent, the operation is limited by the qualities ofthe photoemissive screen. More specifically, because its maximum quantumyield (number of photoelectrons emitted per absorbed quantum) is usuallycon siderably less than thirty percent, the overall gain of the deviceis limited. Moreover, due to the physical nature of the photoemissiveprocess, satisfactory operation in the far infra-red region, forexample, at wavelengths greater than one micron has heretofore not beenpossible. In addition, because the screen is very thin, the absorptionof X- ray energy is very small, thus making direct excitation withimages of such radiation impractical.

The shortcomings of these earlier image intensifier tubes are overcomeby means of the present invention which permits the design of a new typeof image intensifier tube capable of much higher gains than theconventional tube. Furthermore, image intensifier tubes con structed inaccordance with the present invention are also capable of operation withinput images in the far infra-red and in the X-ray regions whereas, aswas previously mentioned, the conventional intensifier tube isrestricted to visible and near infra-red radiation. These improvementsin the operation of image intensifier tubes are made possible byapplying the concept of either tunnel emission or internal avalanchingto them which, in turn, involves the use of a new combination ofmaterials and elements.

It is, therefore, an object of the present invention to provide an imageintensifier tube that is capable not only of operating in the visiblelight region but also in the far infra-red and X-ray regions as well.

It is another object of the present invention to provide a sensitiveimage intensifier tube that will operate effectively where the detectionof low-level images of visible light is involved.

It is a further object of the present invention to employ eithertunnel-emission or internal avalanching principles to expand theefiiciency and range of operation of image intensifier tubes.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawing in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe invention.

The drawing itself schematically illustrates a new type of imageintensifier tube based on the concept of the present invention.

Referring now to the drawing, the tube envelope is designated 10, thefaceplate portion of it being designated 10a. On the inside surface offaceplate 10a is a very thin transparent conductive coating 11, such asstannous oxide, over which a photoconductive layer 12 is coated. Thephotoconductive layer is about 1 mil thick or less and while a number ofdifferent photoconductive materials are available for forming such alayer, cadmium sulphide is an example of one such material that may beemployed. Photoconductive layer 12 is covered with an image retainingmosaic of conducting elements 13 which, in turn, are covered with a thininsulating layer 114, the insulating layer being very thin, in theorder, for example, of Angstroms. The conducting elements of the mosaicmay be made of gold. Finally, a very thin metallic conducting film 15,such as gold, is provided on the surface of the insulating layer.

At the other end of tube envelope 10, opposite faceplate Ilia, is thefiat base 1% of the tube which, on its inside surface is coated overwith a cathode luminescent phosphor layer 16, the phosphor layer havingan aluminized coating 17 over it as is shown in the figure.

As is customary with these types of tubes, the inside of the tube isevacuated and contains focusing electrodes 18a and 1812. Alternatively,magnetic focusing may be used. Since the construction and operation offocusing electrodes are very well known in the art, it is deemedsufficient to merely schematically represent them in the figure and tomention that they focus the electrons emitted from the faceplate portionof the tube into an image at the baseplate portion of the tube. Ofcourse, as is well known, focusing electrodes are customarily connectedto sources of potential which are not shown in the figure. However, twovoltage sources, respectively designated 20 and 21, are shown in thefigure, source 20 providing approximately twenty volts between itsterminals and source 21 providing approximately 10 kilovolts between itsterminals.

Before considering the operation of the present invention, it shouldfirst be stated that cold emission involves the phenomena by whichelectrons are emitted into a vacuum when a large electric field isplaced across a thin insulating layer, the reason for this phenomenabeing due, for example, either to a tunneling action or internalavalanching processes. The tunneling of electrons through thininsulating films and the observance of electron emission into a vacuumhas been reported by C. Mead in Journal of Applied Physics, volume 32,pages 646-652, published in 1961. On the other hand, the internalavalanching process is discussed in an article by Messrs. H. Jacobs, J.Freely and F. A. Brand, in Physical Review, volume 88, page 492,published in 1952.

Considering now the operation, the effect of incident radiation at alocal point on photoconductor 12 is to lower its resistance so that morevoltage appears across insulating layer 14 at the corresponding point.As was previously mentioned, the insulating layer is assumed to be verythin, for example, in the order of 100 Angstroms, so that, in accordancewith the principles mentioned, electric current flows through theinsulator. By also making outer conducting metal film 15 very thin, itcan be expected that a substantial fraction of the electrons movingthrough the insulator will be emitted into the vacuum.

Assuming that only about one-tenth of the total number of electronsemerge into the vacuum space, the overall quantum yield of the combinedphotoconductive coldemission cathode can nevertheless be made muchhigher than a photoemissive surface since the quantum yield of numerousphotoconductive materials is many orders of magnitude higher than thatof a photoemitter. At the same time, the use of a photoconductor makespossible operation in the far infra-red region since photoconductorssensitive in this range of the spectrum are existent, such as lead oxideactivated with sulphur. For other applications, em-

bodirnents of the present invention can be used for operation Where thedetection of low-level images of visible light is involved. Also, sincephotoconductive layers of sufficient thickness and high sensitivity toX-rays are available, image intensifier tubes encompassing the presentinvention can be used to convert and to intensify low-level X-rayimages.

Having thus described the invention, what is claimed is:

1. An image intensifier tube comprising: a transparent conductive layercoated on the inside surface of the tube faceplate; a photoconductivelayer over said transparent conductive layer; a mosaic of separate anddistinct conducting elements covering said photoconductive layer; aninsulating layer deposited over said mosaic; a metallic film depositedon the surface of said insulating layer; a phosphor layer deposited onthe inside surface of the tube baseplate; and an aiuminized coating oversaid phosphor layer.

2. An image intensifier tube comprising: a transparent conductive layercoated on the inside surface of the tube faceplate; a photoconductivelayer deposited over said transparent conductive layer; elements forminga plurality of tunnel-emission diodes mounted over said photoconductivelayer; a luminescent screen deposited on the inside surface of the tubebaseplate; and means for focusing electrons tunneling into the vacuum ofthe tube onto the screen.

3. The image intensifier tube defined in claim 2 wherein saidtunnel-emission diode includes a mosaic of thin metallic elementsmounted over said photoconductive layer, an insulating layer over saidmosaic, and a thin metallic film deposited on the surface of saidinsulating layer.

References Cited UNITED STATES PATENTS 2,824,986 2/1958 Rome 250-213 X2,889,488 8/1959 Kalfaian 313-66 2,903,596 9/1959 Reed 250-213 2,929,9353/1960 Lempert a- 250-213 X 2,944,155 7/1960 Moyer 250-213 3,056,0739/1962 Mead 317-234 3,107,303 10/1963 BerkOWitZ 250-213 3,148,297 9/1964Schneeberger et a1. 313-94 3,246,200 4/1966 Kanter 315-94 RALPH G.NILSON, Primary Examiner.

E. STRICKLAND, M. AMBRAMSON,

Assistant Examiners.

1. AN IMAGE INTENSIFIER TUBE COMPRISING: A TRANSPARENT CONDUCTIVE LAYER COATED ON THE INSIDE SURFACE OF THE TUBE FACEPLATE; A PHOTOCONDUCTIVE LAYER OVER SAID TRANSPARENT CONDUCTIVE LAYER; A MOSAIC OF SEPARATE AND DISTINCT CONDUCTING ELEMENTS COVERING SAID PHOTOCONDUCTIVE LAYER; AN INSULATING LAYER DEPOSITED OVER SAID MOSAIC; A METALLIC FILM DEPOSITED ON THE SURFACE OF SAID INSULATING LAYER; A PHOSPHOR LAYER DEPOSITED ON THE INSIDE SURFACE OF THE TUBE BASEPLATE; AND AN ALUMINIZED COATING OVER SAID PHOSPHOR LAYER. 